Wireless Audio Synchronization

ABSTRACT

An audio distribution system includes an audio source; and a plurality of audio playback devices in communication with each other and with the audio source. A group of the audio playback devices are arranged to render audio content provided by the audio source in synchrony. One of the audio playback devices within the group is configured as an audio master which distributes audio content from the audio source to the other audio playback devices within the group, and one of the plurality of audio playback devices, other than the audio master, is configured as a clock master, which distributes clock information that the group of audio playback devices synchronizes to.

RELATED APPLICATIONS

This U.S. Application is a continuation of U.S. application Ser. No.16/587,957 filed Sep. 30, 2019, now U.S. Pat. No. 10,805,728, whichclaims benefit of U.S. application Ser. No. 15/790,465 filed Oct. 23,2017, now U.S. Pat. No. 10,433,057, all of which are titled “WIRELESSAUDIO SYNCHRONIZATION”, the contents of which are incorporated byreference herein in their entirety.

BACKGROUND

This disclosure relates to wireless audio synchronization.

SUMMARY

All examples and features mentioned below can be combined in anytechnically possible way.

In one aspect, an audio distribution system includes an audio source;and a plurality of audio playback devices in communication with eachother and with the audio source. A group of the audio playback devicesare arranged to render audio content provided by the audio source insynchrony. One of the audio playback devices within the group isconfigured as an audio master which distributes audio content from theaudio source to the other audio playback devices within the group, andone of the plurality of audio playback devices, other than the audiomaster, is configured as a clock master, which distributes clockinformation that the group of audio playback devices synchronizes to.

Implementations may include one of the following features, or anycombination thereof.

In some implementations, the system includes an access point, and thesystem is configured to select the clock master from among the pluralityof audio playback devices based on ping times between the audio playbackdevices of the plurality of audio playback devices and the access point.

In certain implementations, the system is configured to select the audiomaster from among the group of audio playback devices prior to theselection of the clock master.

In some cases, the system is configured to exclude the audio master fromconsideration when selecting the clock master.

In certain cases, the system is configured to select the clock masterfrom among the plurality of audio playback devices, excluding the audiomaster, based on the audio playback device that reports the shortestping time with the access point.

In some examples, the system is configured to select the audio masterfrom among the group of audio playback devices after the selection ofthe clock master.

In certain examples, respective clocks on the audio playback devices ofthe group of audio playback devices are synchronized to that of theclock master prior to the selection of the audio master.

In some implementations, the system is configured to exclude the audioplayback device selected to serve as the clock master from considerationwhen selecting the audio master.

In certain implementations, the system is configured such that if theaudio playback device that is selected to serve as the clock master isalso selected to serve as the audio master, then the system will selecta new clock master.

In some cases, respective clocks on the audio playback devices withinthe group of audio playback devices remain synchronized while the systemselects a new clock master.

In certain cases, in response to a loss in a connection between one ormore of the audio playback devices of the group of audio playbackdevices and the clock master, the system is configured to select a newclock master, and the audio playback devices within the group of audioplayback devices are configured to continue synchronized playback whilea new clock master is selected.

In some examples, the respective clocks of the audio playback deviceswithin the group of audio playback devices are synchronized to that ofthe clock master according to a linear time model m(x)+b, where m is arate difference (e.g., a difference in frequency between crystaloscillators) between the clock master's clock and the clock of the audioplayback device that is synchronizing to the clock master's clock; xrepresents the current clock time according the clock of the audioplayback device that is synchronizing to the clock master's clock; and bis an offset between the clock master's clock and the clock of the audioplayback device that is synchronizing to the clock master's clock.

In certain examples, m is a rate difference between the clock master'sclock and the clock of the audio playback device that is synchronizingto the clock master's clock, and b is an offset between the clockmaster's clock and the clock of audio playback device that issynchronizing to the clock master's clock.

In some implementations, m and b are determined by performing a linearfit of collected timestamps from the clock master (i.e., timestampedcommunications received from the clock master) and the clock (a/k/a“local clock”) of the audio playback device that is synchronizing to theclock master's clock over a given window of time.

In certain implementations, once a new clock master is selected, theother audio playback devices within the group of audio playback devicesadjust their clocks to that of the new clock master using a heuristicapproach which causes the audio playback devices of the group of audioplayback devices to adjust their respective m and b values based on thenew master clock, while the new clock master reduces its rate to zerosuch that value of m for the new clock master equals 1 and the value ofb for the new clock master equals zero.

In some cases, the system includes a controller that is configured toenable a user to select two or more audio playback devices from theplurality of audio playback devices to form the group of audio playbackdevices.

In certain cases, the system is configured to select the audio playbackdevice from the group of audio playback devices that a user selected asa basis for the group of audio playback devices to be the audio master.

In some examples, the clock master is not one of the audio playbackdevices in the group of audio playback devices.

In certain examples, the clock master is one of the audio playbackdevices in the group of audio playback devices.

In some implementations, the system is configured to exclude any audioplayback devices that are not within the group of audio playback devicesfrom consideration when selecting the clock master.

Implementations may provide one or more of the following advantages.

In some implementations, the separation of audio and clock masters canhelp to eliminate the need for resynchronization when the audio masterchanges.

In certain implementations, the separation of audio and clock masterscan help to avoid an interruption of audio playback during clock masterrecovery (on power loss).

In some cases, the clocks of networked audio playback devices can bepre-synchronized to reduce a delay before the start of audio playback.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of an audio distribution system.

FIG. 2 is a schematic diagram of an audio playback device from thesystem of FIG. 1.

FIG. 3 is functional block diagram of a grouping of audio playbackdevices from the system of FIG. 1 arranged for synchronized playback ofaudio.

FIG. 4 is a schematic block diagram of functional aspects of an audioplayback device from the system of FIG. 1.

FIG. 5 is a flowchart of an example process for synchronizing audioplayback among a group of audio playback devices.

FIG. 6 is a flowchart of another example process for synchronizing audioplayback among a group of audio playback devices.

DETAILED DESCRIPTION

Some known whole home audio systems consist of a plurality of audioplayer units, which may be arranged in various rooms throughout a home,and which communicate with each other over a local area network (LAN).Generally, one or more of the player units has access to a source ofaudio content, which may be a source available over a wide area network(WAN), such as an Internet radio station. Typically, two or more of theindividual units can be grouped together, at a user's discretion, intowhat is often referred to as a zone, in which the grouped player unitswithin the zone playback (render) audio content in synchrony.

To facilitate synchronized playback, one of the units (a master unit)will distribute audio content along with timing (clock) information thatone or more of the other units (slave units) use to render the contentin lock step with the master. Each of the slave devices can adjust itsrespective clock, or a rendering time prescribed to the audio content,based on the clock time provided by the master device (i.e., themaster's clock time). This can allow the plurality of player units, eachwith its own clock, to synchronize the rendering of the audio content.Thus, these existing systems rely on a single master unit to serve asboth an audio master as well as a clock master.

This disclosure is based on the realization that it can be beneficial toseparate the roles of the audio master and the clock master.

Audio distribution system 100, FIG. 1, can be used to accomplish amethod for distributing audio data to, and synchronizing audio dataamong, a plurality of audio playback devices (e.g., wireless speakers)that are connected to a network. The system 100 is adapted to deliverdigital audio (e.g., digital music) and includes a number of audioplayback devices 110-1-110-n (collectively referenced as 110). In onenon-limiting embodiment, the audio playback devices 110 are identicaldevices that each include a digital to analog converter that can receivedigital audio signals and convert them to analog form. The audioplayback devices 110 also include an electro-acoustic transducer thatreceives the analog audio signals and transduces them into sound. Theaudio playback devices also include a processor. The audio playbackdevices are connected to one another and also connected to localrouter/access point 114 via a network 116. The audio playback devicesare thus able to communicate with one another. The network 116 can be awired and/or wireless network, and can use known network connectivitymethodologies. The network 116 is part of a local area network (LAN) 118which is connected to a wide area network (WAN) 120, in thisnon-limiting example by connection to Internet 122. The LAN 118 alsoincludes one or more separate computing devices 124 and one or moreseparate local digital audio sources 130. In this non-limiting example,the computing devices include a personal computer 126 and a mobilecomputing device 128 such as a smart phone, tablet or the like. The WAN120 includes server 140 and Internet radio service 142 which can bothcommunicate with the LAN via Internet 122.

One use of the system 100 is to play digital audio data, including butnot limited to an audio stream, over one or more of the audio playbackdevices 110. The sources of digital audio provide access to content suchas audio streams that move over network 116 to the audio playbackdevices. The sources of such audio streams can include, for example,Internet radio stations and user defined playlists. Each of such digitalaudio sources maintains a repository of audio content which can bechosen by the user to be played over one or more of the audio playbackdevices. Such digital audio sources can include Internet-based musicservices such as Pandora®, Spotify® and vTuner®, for example. Networkattached storage devices such as digital audio source 130, and mediaserver applications such as may be found on a mobile computing device,can also be sources of audio data. In a non-limiting example, the userselects the audio source and the playback devices via a user interfaceprovided by a PC 126 and/or a mobile device 128.

FIG. 2 illustrates an exemplary audio playback device 110 as an exampleof this disclosure. The audio playback device 110 includes an enclosure210 (a/k/a “housing”). On the enclosure 210 there resides a graphicalinterface 212 (e.g., an OLED display) which can provide the user withinformation regarding currently playing (“Now Playing”) music. There areone or more electro-acoustic transducers 215. The audio playback device110 also includes a user input interface 216. The user input interface216 can include a plurality of preset indicators, which can be hardwarebuttons. The preset indicators can provide the user with easy, one pressaccess to entities assigned to those buttons.

The audio playback device 110 also includes a network interface 220, aprocessor 222, audio hardware 224, power supplies 226 for powering thevarious components, and memory 228. Each of the processor 222, thegraphical interface 212, the network interface 220, the audio hardware224, the power supplies 226, and the memory 228 are interconnected usingvarious buses, and several of the components may be mounted on a commonmotherboard or in other manners as appropriate.

The network interface 220 provides for communication between thewireless speaker package 200 and audio sources and other networkedwireless speaker packages and other audio playback devices via one ormore communications protocols. The network interface 220 may provideeither or both of a wireless interface 230 and a wired interface 232.The wireless interface 230 allows the wireless speaker package 200 tocommunicate wirelessly with other devices in accordance with acommunication protocol such as IEEE 802.11b/g/n/ac. The wired interface232 provides network interface functions via a wired (e.g., Ethernet)connection.

In some cases, the network interface 220 may also include a networkmedia processor 234 for supporting Apple AirPlay® (a proprietaryprotocol stack/suite developed by Apple Inc., with headquarters inCupertino, Calif., that allows wireless streaming of audio, video, andphotos, together with related metadata between devices). For example, ifa user connects an AirPlay® enabled device, such as an iPhone or iPaddevice, to the network, the user can then stream music to the networkconnected audio playback devices via Apple AirPlay®. Notably, the audioplayback device can support audio-streaming via AirPlay® and/or DLNA'sUPnP protocols, and all integrated within one device.

All other digital audio coming from network packets comes straight fromthe network media processor 234 through a USB bridge 236 to theprocessor 222 and runs into the decoders, DSP, and eventually is playedback (rendered) via the electro-acoustic transducer(s) 215. The networkinterface 220 can also include a Bluetooth circuitry 238 for Bluetoothapplications (e.g., for wireless communication with a Bluetooth enabledaudio source such as a smartphone or tablet).

Streamed data passes from the network interface 220 to the processor222. The processor 222 can execute instructions within the wirelessspeaker package (e.g., for performing, among other things, digitalsignal processing, decoding, and equalization functions), includinginstructions stored in the memory 228. The processor 222 may beimplemented as a chipset of chips that include separate and multipleanalog and digital processors. The processor 222 may provide, forexample, for coordination of other components of the audio playbackdevice 110, such as control of user interfaces, applications run by theaudio playback device 110.

The processor 222 provides a processed digital audio signal to the audiohardware 224 which includes one or more digital-to-analog (D/A)converters for converting the digital audio signal to an analog audiosignal. The audio hardware 224 also includes one or more amplifierswhich provide amplified analog audio signals to the electroacoustictransducer(s) 215 for playback. In addition, the audio hardware 224 mayinclude circuitry for processing analog input signals to provide digitalaudio signals for sharing with other devices.

The memory 228 may include, for example, flash memory and/ornon-volatile random access memory (NVRAM). In some implementations,instructions (e.g., software) are stored in an information carrier. Theinstructions, when executed by one or more processing devices (e.g., theprocessor 222), perform one or more processes, such as those describedelsewhere herein. The instructions can also be stored by one or morestorage devices, such as one or more computer- or machine-readablemediums (for example, the memory 228, or memory on the processor). Theinstructions may include instructions for performing decoding (i.e., thesoftware modules include the audio codecs for decoding the digital audiostreams), as well as digital signal processing and equalization.

FIG. 3 is a schematic illustration of a playback group 300 formed from aplurality of the audio playback devices 110-1, 110-2, 110-3 in the audiodistribution system of FIG. 1. In the illustrated arrangement, one ofthe audio playback devices (audio playback device 110-1 in this case)functions as a master audio device and the other audio playback devicesin the group 300 (i.e., audio playback devices 110-2 & 110-3) functionas audio slaves.

The master device 110-1 receives audio data 301 from an audio source 302(i.e., one of 130 or 142 of FIG. 1) and distributes it to the slavedevices 110-2 and 110-3. In this non-limiting example, such audiodistribution can be by WiFi via wireless access point/router (item 114,FIG. 1). Alternatively or additionally, the audio distribution can be byway of a wired (e.g., Ethernet) connection, or a combination of wiredand wireless connections. Each of the audio devices 110-1, 110-2, and110-3 in the group 300 will play the audio. The audio replay among thedevices in the group is synchronized such that they all play the sameaudio at the same time.

To help ensure that the playback of the audio content is and remainssynchronized, the respective internal clocks (a/k/a “local clocks”) ofthe individual audio playback devices within the group 300 aresynchronized. In principle, such clocks comprise an oscillator and acounter. During synchronized (a/k/a “multi-room”) playback, a protocol,such as Network Time protocol (NTP), or the like, is utilized todistribute timing information in a clock master-slave relationship tokeep the current clock time on all the devices in the group 300synchronized.

The time synchronization protocol is separate and aside from the audiostream. In that regard, one of the audio playback devices (audioplayback device 110-4 in this case) is designated as a clock master.Notably, the clock master is intentionally a different device than theaudio master, and, in the illustrated example, is not even a member ofthe playback group 300 (i.e., in the illustrated implementation, theaudio playback device 110-4 does not render audio in synchrony with thedevices in the group 300).

The clock master provides clock data 303 (i.e., the clock master acts asa time server) to the audio playback devices 110-1, 110-2, 110-3 in thegroup 300 (a/k/a “clock slaves”), which then use that clock data toupdate their respective clocks to synchronize with that of the clockmaster. The clock data may be provided periodically, e.g., every 1 to 6seconds, to keep the grouped devices updated and in sync with the clockmaster. Separately, the audio master may also provide a “play at” time304; i.e., an identification of when all the devices in the group shouldbegin playing the distributed audio content. The “play at” time 304 mayrepresent a clock time at which the units are to render a first samplein a series of ordered audio data samples, with the remainder of theaudio data samples to be played in order at a defined sample rate. The“play at” time 304 is communicated in control data that is separate fromthe audio stream and, in some cases, is only sent once for each track(i.e., it is not included with every frame). Every new track or streamwill get a new “play at” time.

The audio slaves receive the first sample in an audio stream 301 andbegin playback designated “play at” time 304. Since all the groupeddevices are synced to the clock master, and, thus, all have the samecurrent clock time, they all begin playback at the same time. Fromthere, the devices can all provide playback at a constant sample rate,and, consequently, stay in sync.

With reference to FIG. 4, each audio playback device 110 includes aparser 400; a ring buffer 402; a decoder 404; a sample buffer 406; asynchronization module 408; and an asynchronous sample rate converter(ASRC) 410. These components may be in addition to the componentsillustrated in FIG. 2 or may be 25 included in, for example, theprocessor 222, audio hardware 224, and/or memory 228 illustrated in FIG.2. At the beginning of a stream, the data (e.g., encoded audio) startsto flow to the master audio playback device (a/k/a “audio master”) whereit is parsed by the audio master's parser 400 to identify frameboundaries. The parser 400 strips away any container (e.g., MP3) thatencoded audio is packed in and puts it into custom audio frames. Theparsed but still encoded data is stored in the audio master's ringbuffer 402. Next, the encoded data is decoded and a time offset isgenerated and affixed to the header of the audio frame and the decodedaudio frames are stored in the sample buffer 406. The offset representsa time difference between the time when playback of the correspondingframe should start and the “play at” time. The offset is used by thesynchronization module 408 to determine when the audio samples from thecorresponding audio frame are fed into the ASRC 410. The ASRC 410ensures a constant sample-rate for rendering.

For synchronized playback, the encoded data is immediately pulled out ofthe audio master's ring buffer 402 and is provided to the slave playbackdevice(s) (a/k/a audio slave(s)) ring buffer 402. This distribution ofencoded audio data may take place via unicast communications between theaudio master and each of the individual audio slave devices. From there,the audio slaves follow the same process as outlined above with respectto the audio master. Each audio slave will decode the encoded audiopulled from the audio master, assign an offset to the frame header, andstore the decoded audio frames in their respective sample buffers 406.The audio slaves each apply their own offsets to the audio frames, butthese offsets will be the same as those applied by the audio mastersince each device is receiving the same stream and is using the samedecoder software.

Since the clocks on the audio master and audio slaves are in sync andthe offsets are all the same, each device in the group (e.g., group 300,FIG. 3) will feed the same data into its ASRC 410 at the same time. TheASRCs 410 ensure that each device outputs the audio at the same,constant rate. The oscillators on the individual devices may spin atdifferent rates, which could lead to time drift among the devicesdespite the timing synchronization. Synchronization adjustments to theclock time may cause the duration of audio that the corresponding slavedevice needs to play to stay in sync to either grow or shrink. The ASRCs410 on each device account for these time adjustments and manipulate thereceived audio data to ensure a constant sample output rate.

FIG. 5 is a flowchart of an example process 500 for assignment of roles(i.e., master/slave roles) during group formation. In someimplementations, the process 500 can be executed, at least in part, bythe processor 222 (FIG. 2) of one or more of the audio playback devicesexecuting instructions stored in memory 228 (FIG. 2). Operations of theprocess 500 include receiving user instructions regarding the formationof a group (502). In that regard, a group is typically formed via inputin a user interface provided on a controller device, such as the PC 126or mobile device 128 of FIG. 1. The user selects a first device (i.e., afirst one of the audio playback devices 110) to serve as the basis for agroup, and then grows the group by selecting, via the user interface,other ones of the audio playback devices to join the group.Alternatively or additionally, in some implementations, the user canselect multiple devices at once to create a group. Also, in someimplementations, there can be a persistent group or construct, that canbe saved and later selected for playback. For example, in someimplementations, a predefined group may be selected via a voicedirective, e.g. “play music downstairs” as a voice directive thatautomatically starts playing music from a preset (“persistent”) group.Information regarding the group selection is transmitted to, andreceived (502) by one or more the audio playback devices.

The process 500 also includes the selection of an audio master device(504). In some instances, the audio master is automatically set to theaudio playback device that was selected to serve as the basis for thegroup. Alternatively, the device within the group that has the strongestWiFi signal strength may be selected as the audio master. For example,each of the audio playback devices in the group may communicate itssignal strength to the other group members. Once each of the devices inthe group has received the signal strength information from each of theother group members, each device will identify the group member with thehighest/strongest signal strength as the audio master for the group.Alternatively or additionally, the group members may all report theirrespective signal strengths to one network device (e.g., the controllerdevice) and that network device may identify the audio master to thegroup members.

To ensure that the audio output from the audio playback devices is trulysynchronized, the respective clocks of the audio playback devices areall synchronized. To achieve this clock synchronization, one of theaudio playback devices is selected to serve as a clock master (506). Theclock master distributes a reference time that all the audio playbackdevices in the group synchronize their respective clocks to.

The clock master may be one of the audio playback devices in the group,or may be an audio playback device outside of the group; however, theclock master is selected such that no audio playback device canconcurrently serve as both the audio master and the clock master. Inthat regard, the clock master can be selected according to a leaderselection algorithm (a/k/a “leader election algorithm” or “leaderelection”) that excludes the audio master from consideration. Indistributed computing, leader election is the process of designating asingle node as the organizer of some task distributed among severalcomputers (nodes), which in this case are the audio playback devices110. Before the task is begun, all network nodes are either unawarewhich node will serve as the “leader” (or coordinator) of the task, orunable to communicate with the current coordinator. After a leaderelection algorithm has been run, however, each node throughout thenetwork recognizes a particular node as the task leader.

The network nodes communicate among themselves to decide which of themwill get into the “leader” state. For that, they need some method tobreak the symmetry among them. For example, if each node has unique andcomparable identities, then the nodes can compare their identities, anddecide that the node with the highest identity is the leader. In somecases, the leader selection algorithm can select the clock master to bethe playback unit that reports the lowest ping time.

For example, each of the audio playback devices in the group may pingthe access point (item 114, FIG. 1) and communicate its ping time (i.e.,a round-trip time for messages sent from the audio playback device tothe access point that are echoed back to the audio playback device) tothe other group members. Once each of the devices in the group hasreceived the respective ping times from each of the other group members,each device will identify the group member (excluding the audio master)with the shortest ping time as the clock master for the group.Alternatively, as mentioned above, the process of selecting the clockmaster can be extended to include all the audio playback devices in asystem (such as system 100, FIG. 1), and, thus, is not necessarilylimited to only the members of a playback group. In which case, theclock master can be selected from the system device (excluding the audiomaster) that reports the lowest ping time, whether or not it is part ofthe playback group.

In some implementations, the audio playback devices may be arranged in apeer-to-peer network that includes the access point. In suchconfigurations, the clock master may be selected to be the audioplayback device that records the shorted ping time with the accesspoint, as described above, or, alternatively, the clock master may bedetermined based on ping times among the nodes of the peer-to-peernetworks. For example, the clock master may be selected to be the audioplayback device (e.g., excluding the audio master) that records thesmallest mean ping time with the other network nodes.

With a clock master selected, the members of the group, including theaudio master, can begin to synchronize (508) the clock master's clock.In some cases, the clocks of the audio playback devices can besynchronized to the master clock time according to a linear time modelm_(n)x+b_(n), where m_(n) is the rate difference between the master'sclock and the clock of slave_(n), x represents the current clock time onslave_(n), and b_(n) is the offset between the master clock and theclock of slave_(n) since booting up. In some cases, m_(n) and b_(n) aredetermined by performing a linear fit of collected timestamps from clockmaster and local clock over a given window of time. With this model inplace, the devices can maintain some degree of synchronization even inthe event that the clock master needs to be changed.

In that regard, there are two situations in which a new clock master maybe required: 1.) if the current clock master loses power; or 2.) if theclock master becomes the audio master—such as may occur when the currentclock master is selected as the head of a new grouping. In either case,the leader selection algorithm restarts. At this point, the system is inan “open loop” state in which the audio playback devices initiallymaintain clock synchronization using their existing, respective m_(n),and b_(n) values. Then, once a new clock master is selected, the otheraudio playback devices start slowly adjusting their clocks to that ofthe new clock master using a heuristic approach which causes the audioplayback devices to adjust their respective m and b values based on thenew master clock, while the new clock master slowly reduces its rate tozero, such that m_(master)=1, and b_(master)=0.

Referring to FIG. 6, in an alternative embodiment (600), the clockmaster can be selected (602) before a group is formed (606) and/orbefore an audio master is selected (608). This can beneficially allowthe audio playback devices to be clock synchronized (“pre-synced”) (604)before audio content is selected for playback, which can ultimatelyallow for a shorter delay before playback begins (610). In thisembodiment, the clock master can be selected using a similar leaderselection algorithm to that described above, but without having toexclude the audio master. The audio master can then be determined atsome later time. In that regard, the audio master can be selected by amaster selection algorithm that excludes the clock master fromconsideration.

The master selection algorithm may select (608) the audio master basedon the audio playback device within a playback group that reports thestrongest WiFi signal connection, excluding any device that is servingas the clock master. Alternatively, the audio master may be selected(608) based on the audio playback device within a playback group thatreports the strongest WiFi signal connection, regardless of whether thatdevice is also serving as the clock master. In which case, if the clockmaster is subsequently selected (608) as the audio master, then a newclock master can be selected while or after playback of the selectedaudio begins—that is, the audio playback devices within the group canstill take advantage of the fact that their clocks were pre-synchedwhile a new clock master is selected.

While implementations have been described in which an audio playbackdevice may not contemporaneously serve as both an audio master and aclock master for a playback group, it is contemplated that an audioplayback device may serve as a clock master for one playback group,while also serving as an audio master for another, different playbackgroup within the same audio distribution system.

In some implementations, the audio source is connected to one of theaudio playback devices of a playback group via a personal area network(PAN) (e.g., via a Bluetooth connection). In such cases, the device thatis coupled to the audio source can be selected as the audio master—thatselection can then be communicated from the audio master to the otherdevices in the group via the LAN.

In certain implementations, the audio source is connected to one of theaudio playback devices of a playback group via a hardwire connectionthrough an auxiliary input port on the audio playback device. In suchcases, the device that is directly coupled to the audio source can beselected as the audio master—that selection can then be communicatedfrom the audio master to the other devices in the group via the LAN.

Implementations of the systems and methods described above comprisecomputer components and computer-implemented steps that will be apparentto those skilled in the art. For example, it should be understood by oneof skill in the art that the computer-implemented steps may be stored ascomputer-executable instructions on a computer-readable medium such as,for example, floppy disks, hard disks, optical disks, Flash ROMS,nonvolatile ROM, and RAM. Furthermore, it should be understood by one ofskill in the art that the computer-executable instructions may beexecuted on a variety of processors such as, for example,microprocessors, digital signal processors, gate arrays, etc. For easeof exposition, not every step or element of the systems and methodsdescribed above is described herein as part of a computer system, butthose skilled in the art will recognize that each step or element mayhave a corresponding computer system or software component. Suchcomputer system and/or software components are therefore enabled bydescribing their corresponding steps or elements (that is, theirfunctionality), and are within the scope of the disclosure.]

A number of implementations have been described. Nevertheless, it willbe understood that additional modifications may be made withoutdeparting from the scope of the inventive concepts described herein,and, accordingly, other implementations are within the scope of thefollowing claims.

What is claimed is:
 1. A system including a plurality of audio playbackdevices in communication with each other, the system comprising: a firstaudio playback device of the plurality of audio playback devices,wherein the first audio playback device is configured to distributeaudio content to at least one other audio playback device of theplurality of audio playback devices; and a second audio playback deviceof the plurality of audio playback devices, the second audio playbackdevice different from the first audio playback device, wherein thesecond audio playback device is configured to distribute clockinformation to at least one other audio playback device of the pluralityof audio playback devices, and wherein the clock information is used tohelp determine playback timing for the audio content.
 2. The system ofclaim 1, wherein the at least one other audio playback device that isconfigured to receive audio content from the first audio playback deviceincludes the second audio playback device.
 3. The system of claim 1,wherein the at least one other audio playback device that is configuredto receive clock information from the second audio playback deviceincludes the first audio playback device.
 4. The system of claim 1,wherein the at least one other audio playback device that is configuredto receive audio content from the first audio playback device is in agroup with the first audio playback device, and wherein the group isconfigured to render the audio content in synchrony based at least inpart on the clock information.
 5. The system of claim 4, wherein the atleast one other audio playback device that is configured to receiveaudio content from the first audio playback device includes the secondaudio playback device.
 6. The system of claim 4, wherein the at leastone other audio playback device that is configured to receive clockinformation from the second audio playback device includes the firstaudio playback device, and wherein the second audio playback device isoutside of the group.
 7. The system of claim 1, wherein the second audioplayback device is selected according to an algorithm that excludes thefirst audio playback device from consideration.
 8. The system of claim1, wherein the second audio playback device is configured to distributeclock information to the at least one other audio playback device beforethe first audio playback device is configured to distribute audiocontent to the at least one other audio playback device.
 9. The systemof claim 1, further comprising a third audio playback device of theplurality of audio playback devices, the third audio playback devicedifferent from the first and second audio playback devices, wherein inresponse to the second audio playback device losing power or beingconfigured to distribute audio content to at least one other audioplayback device of the plurality of audio playback devices, the secondaudio playback device is no longer configured to distribute the clockinformation and the third audio playback device is configured todistribute the clock information.
 10. The system of claim 1, wherein thefirst audio playback device is further configured to, in addition todistributing audio content to at least one other audio playback device,distribute an identification of when the audio content should be played.11. A method of distributing audio content to and synchronizing audiodata among a plurality of audio playback devices that are incommunication with each other, the method comprising: distributing, by afirst audio playback device of the plurality of audio playback devices,audio content to at least one other audio playback device of theplurality of audio playback devices; and distributing, by a second audioplayback device of the plurality of audio playback devices, the secondaudio playback device different from the first audio playback device,clock information to at least one other audio playback device of theplurality of audio playback devices, wherein the clock information isused to help determine playback timing for the audio content.
 12. Themethod of claim 11, wherein the at least one other audio playback devicethat receives audio content from the first audio playback deviceincludes the second audio playback device.
 13. The method of claim 11,wherein the at least one other audio playback device that receives clockinformation from the second audio playback device includes the firstaudio playback device.
 14. The method of claim 11, wherein the at leastone other audio playback device that receives audio content from thefirst audio playback device is in a group with the first audio playbackdevice, and wherein the group is configured to render the audio contentin synchrony based at least in part on the clock information.
 15. Themethod of claim 14, wherein the at least one other audio playback devicethat receives audio content from the first audio playback deviceincludes the second audio playback device.
 16. The method of claim 14,wherein the at least one other audio playback device that receives clockinformation from the second audio playback device includes the firstaudio playback device, and wherein the second audio playback device isoutside of the group.
 17. The method of claim 11, further comprisingselecting the second audio playback device according to an algorithmthat excludes the first audio playback device from consideration. 18.The method of claim 11, wherein the second audio playback devicedistributes clock information to the at least one other audio playbackdevice before the first audio playback device distributes audio contentto the at least one other audio playback device.
 19. The method of claim11, further comprising selecting a third audio playback device of theplurality of audio playback devices to distribute the clock informationto the at least one other audio playback device, the third audioplayback device different from the first and second audio playbackdevices, wherein the selecting is in response to the second audioplayback device losing power or being configured to distribute audiocontent to at least one other audio playback device of the plurality ofaudio playback devices.
 20. The method of claim 11, further comprisingdistributing, by the first audio playback device, in addition todistributing audio content to at least one other audio playback device,an identification of when the audio content should be played.